\begin{pspicture}(0,-.5)(7,7)
  \input{fig/1d-helpers.tex}
	% Box around 2x2 cell.
	\psframe*[linecolor=lightgray,fillcolor=lightgray]
					(1.6,2.6)(5.4,6.4)
	% Dots and arrows.
	\multiput(1,0)(2,0){2}{
		\multiput(0,1)(0,2){2}{
			\Hright01\Eright12
			\Hleft21\Eleft32
			\Eup10\Hup21
		}
	}
	% Dashed arrows.
	\psset{linestyle=dashed}
  \Eleft23\Hright54  
	% Axes.
	\psset{linestyle=solid}
	\Taxis016\Xaxis160  
\end{pspicture}
